The most popular theory behind April Fool's Day is the change from Julian to Gregorian calendar.
Originally, the Roman New Year fell on April 1. When it was changed to January 1st, anyone who celebrated it was laughed and played tricks upon.
The moral of the possible history lesson? It's best to base our dates and deeds from the Book of the Creator of time and good works!
And that's a fact. Source: Where Does April Fool's Originate?